1. Field of the Invention
The present invention relates to a power supply that generates a voltage by driving a piezoelectric transducer, to an image forming device including the power supply, and to a method of controlling the piezoelectric transducer.
2. Description of the Related Art
A piezoelectric transducer (a piezoelectric resonator such as a ceramic plate, for example) can function as a voltage converter that converts an input alternating current (ac) voltage to a boosted output voltage. Such piezoelectric transducers are widely used in the power supplies of image forming devices to generate, for example, driving voltages for cold cathode tubes in liquid crystal displays, or voltages supplied to the transfer and developing rollers in electrophotographic devices. The output characteristics (resonance characteristics) of piezoelectric transducers vary with factors such as the load impedance, e.g., the impedance of the cold cathode tube or transfer roller. To stabilize the output voltage, it is necessary to control the frequency of the ac voltage supplied to the piezoelectric transducer (the driving frequency) according to load impedance variations and other factors. Control of the driving frequency can be implemented by an analog circuit such as a voltage controlled oscillator (VCO). A power supply unit using a VCO is disclosed by Uchiyama et al. in Japanese Patent Application Publication No. 2007-189880.
A problem with the power supply disclosed in Japanese Patent Application Publication No. 2007-189880 is that since it uses analog control of the driving frequency, it has a large number of analog circuit components. Another problem is that the piezoelectric transducer has resonant frequencies (referred to below as spurious frequencies) other than the natural resonant frequency used for voltage boosting, and generates excessive heat when driven at or near a spurious frequency. To avoid overheating, it is desirable to control the driving frequency so as to avoid these spurious frequencies, but it is difficult to configure an analog control circuit for a VCO to avoid such spurious frequencies in a flexible way.
A recently proposed solution to these problems is to use a digital circuit to control the driving frequency. In Japanese Patent Application Publication No. 2010-148321, for example, Kosake et al. disclose a power supply apparatus using digital control of the driving frequency of the piezoelectric transducer and an image forming device including the power supply apparatus.
The disclosed power supply apparatus sets a starting frequency fstart between the spurious frequencies and the resonant frequency f0, (f0<fstart<spurious frequencies), and avoids the spurious frequencies by keeping the driving frequency between the starting frequency fstart and the resonant frequency f0.
These conventional power supplies, however, are problematic in that their starting output voltages are not low enough. More specifically, the absolute values of their starting output voltages are not low enough to be used for warmup purposes in electrophotographic image forming devices. Warmup is necessary because the voltage boosting ratio of a piezoelectric transducer (the ratio of its output voltage amplitude to its input voltage amplitude) is temperature dependent. The ratio is low at low temperatures, so for a while after the image forming device is powered on, the piezoelectric transducer may need to be warmed up by driving it in an idling mode, to raise its temperature and stabilize its input-output characteristic. If the output voltage generated during the warmup period is supplied to a transfer roller, however, the transfer roller draws residual toner from the surface of the facing photosensitive drum onto the transport belt. The residual toner is then removed from the transport belt by a cleaning device and accumulates in a collection receptacle. The higher the output voltage is during the warmup period, the faster the collection receptacle fills up and the more often it has to be replaced. This raises a problem in terms of environment-friendly product design, a topic of concern in recent years.